The present invention relates to an image reading apparatus used for reading images for medical use in which: an X-ray image for medical use recorded on a recording medium is subjected to 2-dimensional optical scanning using a light beam; transmitted light and stimulatively emitted light is converted into an electric signal; and the thus obtained electric signal is subjected to analog-digital conversion and sent out to a computer as image data.
In the case of taking an X-ray image for medical use, in order to remove X-rays diffused in the body of an patient, a filter referred to as a grid is frequently used. Therefore, an image of the grid is photographed on the X-ray image when the grid is used. The grid image is composed of longitudinal stripes disposed at regular intervals. Examples of the density of the stripes are: 34, 40, and 80 pieces/cm. That is, the intervals of the longitudinal stripes are various. When an image formed on a recording medium on which the grid is photographed is subjected to optical scanning so as to digitalize the image data, the following phenomenon appears. According to a relation between a sampling spatial frequency (F.sub.s) corresponding to the frequency of analog-digital conversion and a grid spatial frequency (F.sub.g) appearing when the grid image is optically scanned, aliasing is caused in the frequency region, so that a low frequency component, which is not on the original image, appears on the digital image. This is referred to as moire artifacts caused by aliasing. When a digitalized medical image is outputted onto CRT or film so as to make medical diagnosis, these moire strips are obstacles to success of medical diagnosis. FIGS. 9(a) to 9(d) are views showing a mechanism of generation of moire artifacts. In order to prevent the generation of moire patterns, F.sub.s and F.sub.g must satisfy the following inequality. EQU F.sub.s &gt;2.multidot.F.sub.g ( 1)
The above is a well-known fact referred to as Nyquist's Sampling Theory. In order to realize this, for example, the following technique is disclosed in Japanese Patent Publication No. 198265/1990.For the purpose of satisfying the Nyquist Sampling Theory, an electric analog filter is applied to the image reading apparatus so that the maximum spatial frequency component (F.sub.max) in the image signal can satisfy the following equation. EQU F.sub.max =F.sub.s /2 (2)
As a result, after the amplitude of the grid image signal has been reduced, the A/D-conversion is carried out.
Recently, an image reading apparatus has been developed in which the pixel size can be variously changed in the process of digitalization. In this type of apparatus, F.sub.s can be made variable. In accordance with the above expression (2), it is necessary that the cut-off characteristic of the analog filter is made to be variable according to each F.sub.s. In the case of using an analog filter, for example, it is proposed that R and C in the apparatus are made to be variable and the cut-off frequency is controlled in accordance with the setting of CPU.
Problems caused in the above system are described as follows.
(1) Since this filter is realized in the analog circuit, the filter characteristic is changed in accordance with the environmental temperature and humidity. It is necessary to provide a complicated mechanism for the compensation of the change in the environmental temperature and humidity. Therefore, the reliability of the apparatus is deteriorated and further the cost of the apparatus is raised.
(2) Since the cut-off characteristic of an analog filter is very gentle, in order to make the amplitude at F.sub.g to be close to 0, it is necessary that the cut-off frequency (F.sub.c) of the filter is set at a point considerably lower than F.sub.g. FIGS. 10(a) to 10(c) are views showing the spectrum of an image signal obtained when an analog filter is used. When the analog filter shown in FIG. 10(b) is applied to the image shown in FIG. 10(a) in which the grid has been photographed, an amplitude of the image signal having no influence on moire artifacts is also suppressed. As a result, high frequency components of a digitalized image are reduced, so that the image sharpness is deteriorated as illustrated in FIG. 10(c).
There are two causes of moire stripes. One is aliasing of the spectrum described above, and the other is a phenomenon referred to as a beat. The phenomenon of a beat occurs when the following condition is satisfied. EQU F.sub.g .apprxeq.F.sub.s /2 (3)
As illustrated in FIG. 11(a), the amplitude of a digitized image is different according to a case in which the image signal is sampled at a position close to the top of the amplitude of the grid image and also according to a case in which the image signal is sampled at a position close to the center of the amplitude of the grid image, and as illustrated in FIG. 11(b), the envelope appears as an image of the frequency considerably lower than that of the grid image. According to the image density of the background, the degree of the influence of this phenomenon upon a human's eyes is different. In general, in a density range in which the background density is 0.6 to 1.2, human eyes are able to recognize the beat phenomenon as a problem. It is impossible to completely solve this problem by performing the filtering expressed by the above expression (2). Further, in the case of an analog filter, because of the above problem (2), it is very difficult to find the optimum condition for removing the beat phenomenon.
When the rotational speed R of the polygonal mirror is R=2000 rpm, the focal distance f of the F.theta. lens is f.theta.=380 mm and the pixel size (P.sub.size) is variable in the range from 100 to 200 .mu.m, the pixel clock F.sub.pix can be expressed by the following expression. ##EQU1##
Accordingly, the pixel clock F.sub.pix is variable in the range from 1.6 to 0.8 MHz. In this connection, the units are as follows. F.sub.pix is Hz!, R is rpm!, f is m!, and P.sub.size is m!. In this embodiment, the frequency that is 16 times as high as the frequency of the pixel clock is generated by PLL circuit in the clock generator (19), and the frequency is divided into 1/16 by the frequency divider, so that the pixel clock can be obtained. In order to make the pixel size variable, a large number of oscillators may be changed over. In the methods described above, the rotational speed of the polygonal mirror is constant, however, it is understood from the expression (4) that the pixel size can be changed even when the rotational speed of the polygonal mirror is variable. It is possible that both the pixel clock and the rotational speed of the polygonal mirror are made to be variable. Commonly, the pixel is square. Therefore, when the pixel size in the primary scanning direction is variable as described above, the medium conveyance speed in the subsidiary direction is changed at the same time, for example, by changing the pulse frequency of a pulse motor. In this way, the pixel size in the subsidiary scanning direction can be made to be variable.
The operation of cut-off of the stationary LPF (14) is fixed at 0.8 Mhz that is 1/2 of the maximum pixel clock (sample frequency) 1.6 MHz. In this case, if the pixel clock is used for A/D conversion as it is, in the case where the sampling is conducted at the minimum pixel clock 0.8 MHz, a turn is generated in the digitalized image data. As a result, there is a possibility that moire strips are caused on the image.
U.S. Pat. Nos. 5,006,708 and 5,028,784 disclose a means for solving the above problems. For example, U.S. Pat. No. 5,006,708 discloses the following means. There is provided a digital filter to cut frequency components higher than the Nyquist frequency (F.sub.s /2) for the purpose of removing noises at least in one direction of the recording sheet. U.S. Pat. No. 5,028,784 discloses the following means. There is provided a digital filter to cut frequency components higher than the turn component and the Nyquist frequency for the purpose of removing the moire caused by the grid at least in one direction of crossing the grid image. However, according to the above proposals, after the image information has been temporarily taken into the memory means, it is subjected to filtering processing. Therefore, it is necessary to provide an apparatus for storing the image information. Accordingly, the processing time is greatly increased. For the above reasons, it is desired to provide an apparatus having a simple structure by which the processing time can be reduced while moire artifacts generated by the grid are removed.